Hi Jean, What I can assume that your embedding your video using a rich-text element from the right-hand side 'Insert Elements' panel of the draft of the landing page. The rich-text does adds ="" to the allowfullscreen attribute. So instead of using it, try to embed your video in an HTML element from that panel. The HTML element won't touch the allowfullscreen attribute. Hope this solves your issue.

So, I filed a ticket to change this but we've learned that allowfullscreen="" is equivalent to allowfullscreen

It won't show you the full-screen button in the LP Previewer but when you approve and deploy the landing page, it will work fine. So, the fact that the syntax is changed actually doesn't impact anything.